The present invention pertains to power steering assemblies for motor vehicles and, more particularly, to a ball nut assembly for a rack electrical power assist steering system.
In a typical rack electrical power assist steering system for a motor vehicle, a motor provides assistance to a driver to help steer the motor vehicle. This assistance is generally most beneficial when a large amount of steering input is needed, e.g., when turning the motor vehicle into a parking space. The driver turns a steering wheel that rotates a shaft to move a rack bar connected to the motor vehicle's wheels to steer the motor vehicle. A pulley system, employing a belt, connects the motor to a ball nut assembly that is used to convert a rotational force provided by the motor into a linear force in the rack bar to reduce the effort needed by the driver to steer the motor vehicle. In connection with the present invention, three criteria were established for evaluating potential ball nut assembly designs. First, a driven pulley of the ball nut assembly can have some axial compliance but must have negligible radial compliance in order to maintain belt tension and avoid belt skip. Second, the ball nut assembly must help maintain the motor rotational position relative to the rack bar's linear position, i.e., there must be no rotational slip between the driven pulley and a ball nut of the ball nut assembly. Third, the ball nut should have some compliance axially and radially in order to be robust to tolerance stack-ups and to transfer load efficiently, i.e., there should be no binding in the ball circuit. Current ball nut assembly designs do not meet at least one of the above criteria. In particular, current single bearing ball nut assembly designs only allow for axial compliance and radial lash and are therefore unforgiving of possible tolerance stack-up issues between interacting components. Potential consequences of this include steering noise, vibration and/or feel issues that are generally deemed unacceptable by the driver.
In view of the above, there is a need in the art for a ball nut assembly that meets each of the design criteria set forth above.